Since the invention of the bicycle in the early 1800's, much effort has been devoted to the attainment of high speeds and high efficiencies on bicycles and other, similar human powered devices. With increasing shortages of energy, there has recently been a great revival of interest in human powered vehicles, particularly bicycles, and much work has been put in developing bicycles which can attain higher speeds and/or require less energy from a rider to propel.
The resistance which a bicycle rider must overcome includes rolling resistance of the bicycle and the wind resistance, or drag, of the bicycle and rider. Of these two factors, wind resistance is the dominating factor at higher speeds.
The wind resistance, or drag of a cycle may be characterized by a drag coefficient, C.sub.d which is defined as: ##EQU1## where F is the drag or aerodynamic force on the cycle, V is the velocity of the cycle, p is the air density, and A is a reference area determined by the area of the frontal projection of the cycle. A conventional bicycle and rider have a frontal area of approximately 4.5 square feet, and the drag coefficient, C.sub.d, is approximately 1. This limits the top speed attainable by such a bicycle to approximately 30 mph over moderate distances on flat surfaces with no tail wind. As a basis for comparison, a typical automobile would have a drag coefficient of about 0.4, while some glider fuselages have drag coefficients as low as 0.034.
In an effort to reduce wind resistance and attain higher speeds, many different designs for low-drag bicycles have been developed. These designs include streamlined fairings and fully-enclosed bodies for conventional two-wheeled bicycles. To reduce frontal area and hence wind resistance, other cycle designs have been developed in which the cyclist is in a recumbant position. This allows the cycle and cyclist to be enclosed by a streamlined body of much lower frontal area.
Wind tunnel testing has been carried out in order to determine the optimum shape of a streamlined cycle. One article which describes such testing is "Streamlining: Designing for Speed" by Fujikawa and Olsen, in the November, 1977 issue of Bicycling. In wind tunnel tests such as those described in this reference, the cycle model is typically mounted on a fixed horizontal surface, simulating the road or other surface on which the cycle is driven. When using such a testing procedure, there is no relative motion between the cycle and the floor; and the air stream thus moves past both the model and the "ground" at the same speed. This is different from the actual situation when a cycle is being ridden in which there is no relative motion between the air and ground, and a cycle moves through the air and over the ground at the same speed, neglecting the effects of wind.
Although it would at first appear simple to conduct wind tunnel simulations with a moving ground surface to approximate the actual operating conditions, this is not the case. Such tests require much more complicated equipment. Additionally, studies of automotive aerodynamics indicate that wind tunnel experiments with moving ground planes are not necessarily more accurate than those with stationary ground planes. In one study described in "Problems of Ground Simulation in Automotive Aerodynamics," by F. N. Beauvais, published by the Society of Automotive Engineers, Report No. 680121, the aerodynamic coefficients predicted with a moving ground plane simulating actual conditions were significantly less accurate than those predicted with a fixed ground plane. Thus little, if any, wind-tunnel testing of low-drag cycle designs has been done under realistic conditions.
A bicycle of practical design is constrained to have the bulk of its body within a few feet of the ground. Additionally, the wheels of the cycle extending to the ground contribute significantly to the aerodynamic drag. It is therefore not surprising that wind tunnel tests using stationary ground surfaces predict that a minimum drag configuration occurs where there is little or no gap between the cycle body and the ground. The most successful streamlined cycles, evaluated in terms of maximum attainable speed, have adopted such a configuration, and these cycles typically have a shape which includes a streamlined, rounded top with vertical walls which extend very near to the ground. Several such streamlined bodies are shown and described in the Summer, 1979, issue of Human Power, including the first human-powered vehicle to exceed 55 mph.
The drag coefficient predicted by such wind tunnel simulations is often much different from the actual drag coefficient achieved by a cycle; and although current streamlined cycle bodies have achieved higher speeds and efficiencies then previous designs, they have still fallen far short of their maximum theoretical performance, due, at least in part, to the above-described difference between actual conditions and the conditions of most wind tunnel simulations. In one typical design, the drag coefficient predicted by wind tunnel testing on a stationary surface was approximately 0.07. Under operating conditions with the full-size cycle, however, the actual drag coefficient was calculated to be approximately 0.2.
In addition to the aerodynamic factors mentioned above, there are many other practical conditions which must be taken into account in designing an efficient, low-drag cycle. First, the cycle body must be able to enclose the cyclist, and the position of the cyclist should be one which is reasonably comfortable and which is conducive to sustained exertion as the cyclist propels the cycle. The cyclist's position and the cycle body design should also be such that the cyclist has good vision both forward and sideways when riding the cycle.
Additionally, a practical cycle requires that a cyclist have quick and ready access to the ground with both feet in order that the cycle may be easily started and stopped. Some high speed cycles have been designed which provide low speed stability by using two wheels to provide stability in the manner of a tricycle. These cycles are impractical for every day transportation use, however, since the ability to lean into a turn is necessary to negotiate all but the most gentle of curves, especially at high speeds. A conventional two-wheeled cycle, on the other hand, is able to negotiate sharp turns at relatively high speeds without difficulty, since a cyclist is able to counteract centrifugal force by leaning into the turn. The ability to lean into a turn also requires that the ground clearance of the streamlined body be such that it does not touch the ground when the cycle is leaned or tilted sideways at angles of 20 or 30 degrees, which are commonly encountered when negotiating turns.
Finally, the cycle should present a small sideways profile so that strong side winds do not overly affect the stability of the bicycle. Also, the cycle streamlined body should be designed such that it may be fabricated with reasonable economy and should be capable of achieving high strength with light weight.
In the present invention, each of the above-stated problems have been solved in a novel and practical manner to provide an efficient and practical human-powered vehicle capable of achieving relatively high speeds.